Berghaus does a small range of Lithium Ion rechargeable power electrically heated jackets and gloves.

http://www.exo2.co.uk/berghaus_products.html#softcell

Combine it with a portable solar panel such as a Sunlinq 25+ Watt folding panel then solar radiation energy collected during the day could be stored and then used during the cold night time hours.

Now if they had a sleeping bag liner, which could measure the internal temperature of the bag and activate the heater pad automatically to regulate the temperature to stop that 4am waking up in the middle of the night cold effect...